A rectangle of material. Most of the towels made pre-collapse have worn away to nothing. Occasionally, looters and scavengers might come across a cache of the heavy, fluffy towels of the pre-fallen world. These are valuable more for the status symbol than any other reason, and are sold to the comparatively wealthy. Most other people make do with scraps of cloth, or towels woven since the fall.
"If you want to survive out here, you always gotta know where your towel is." ~ Ford Prefect
"If you want to survive out here, you always gotta know where your towel is." ~ Ford Prefect